Recording elements for electrostatic printing



United States Patent 3,347,670 RECORDING ELEMENTS FOR ELECTROSTATIC PRINTING George R. Nelson and Lee A. Carlson, Framingham, Mass, assignors to Dennison Manufacturing Company, Framingham, Mass, a corporation of Nevada No Drawing. Filed June 19, 1963, Ser. No. 288,905

3 Claims. (Cl. 961.5)

ABSTRACT OF THE DISCLOSURE A recording element for electrostatic printing comprising a supporting member coated with finely divided, photoconductive particles suspended in a resinous insulating binder of a mixture of a polar and non-polar film forming resin.

charge, followed by exposure to a light image to dischargeexposed surface areas. Visible images are developed on the charged latent image surface by contact with finely divided toner particles carrying electrostatic charge. It is well known to develop the latent images by cascading finely divided colored toners over the image or by contacting the image with charged toner particles suspended in an insulating liquid. The image can be fixed, if desired, either on the latent image surfaces or following transfer to another surface, by heat or solvents.

One type of recording element usable in the foregoing electrostatic printing processes comprises a supporting member, usually paper but which may be metal or any other suitable surface, coated with finely divided, photoconductive particles suspended in a resinous insulating binder. Photoconductive zinc oxide particles are most frequently employed. Recording elements of this type are ordinarily employed for production of a final copy directly on the photoconductive surface and the element is not reused. Consequently, cost is an important consideration.

Numerous resins have been heretofore suggested for use as a binder for the particulate photoconductive particles. It has been heretofore known that certain of these polymers, desirable from the standpoint of cost, do not give as much development as desired, presumably because they do not accept adequate voltage when charged in the dark (or with a safe light). One remedy for this defect for styrene-butadiene copolymers is disclosed in United States Letters Patent No. 3,041,168.

In the art of making copies of documents or the like, for example in the office copying field, the time within which the recording element can be electrostatically charged and discharged with light is limited in order that the machine produce copies rapidly. In one office copying machine for example, charging is accomplished in one second or less in a corona discharge device at 6 kv. potential and the charge dissipated in about 3 to 8 seconds with 10 to 20 foot candles illumination from an incandescent light source. As shown for example in the aforementioned Patent No. 3,041,168, if insuflicient charge is accepted, inadequate development for many purposes is generally obtained. This is evidenced by mottled or nonuniform development most apparent in larger image areas. Conversely, it has now been found that if adequately large voltages are accepted at the charging station, but not adequately discharged under illumination, residual charge exists in background areas which may develop with a resulting loss of contrast.

It is accordingly the principal object of the present in vention to provide improved recording elements of the above type and methods of making the same. Further objects include provision of low cost recording elements which provide better contrast with more uniform image development and less background discoloration.

The foregoing and other objects and advantages are accomplished in accordance with this invention which is based on the discovery that non-polar, film-forming binders, such as the aforementioned styrene-butadiene copolymers and as hereinafter defined, are characterized by rapid discharge upon exposure to light, hence providing background areas relatively free of developer, but also by low charge acceptance productive of undesirably light and non-uniform image areas. Conversely, it has been discovered that polar film-forming binders, as hereinafter defined, are characterized by good charge acceptance but also by poor discharge performance. Accordingly, polar binders produce good image areas but undesirably poor background areas. This invention is based on the further discovery that mixtures of these two classes of materials within proportions defined below will provide both good charge acceptance and good discharge upon exposure to light, with the result that both improved contrast andimage development is obtained. Such mixtures may be intimate physical mixtures of the two types of resinous, film-forming binders or may be copolymers of co-reactive monomers as described below.

As used herein, polar binders are those film-forming resinous materials sufficiently insulating to retain the charge on the photoconductive particles, and which, in film-forming molecular weights, have a dielectric constant of 3.0 or higher. Non-polar binders are those insulating binders which, in film-forming molecular weights, provide a dielectric constant of 2.7 or less. Film-forming is used herein to mean a resin which can be cast, extruded or otherwise formed into a self-supporting film. By a polar or non-polar monomer is meant a monomer which, if polymerized to a homopolymer at film-forming molecular weights, would provide either the polar or non-polar material as defined above in terms of dielectric constant.

In accordance with this invention it has been found that the desirable improvements described herein are obtained if the polar binder comprises from about 33 to about 80% by weight of the total binder present, and more preferably from about 50 to 80% by weight of total binder. Binder modifying agents such as waxes or plasticizers, if employed, should be included in determining the above percentages. However, these modifying materials are preferably not used. The amount of photoconductive particulate matter to be suspended in the binder is within previously known limits, generally from about 50 to 95% by weight of the total coating and preferably or higher.

This invention may be better understood by reference to the following example wherein all parts are by weight unless otherwise indicated.

EXAMPLE 1 Zinc oxide, Florence green seal No. 8 117 Styresol 4445 23 Pliolite SSD 10 Toluene 75 sensitizing dye mixture ml 5.4

The resins and dye mixture were dissolved in the solvent and the finely divided zinc oxide suspended therein.

Styresol 444.5 is a polar, styrenated-alkyd resin obtained.

from the Reichold Chemical Company, and Pliolite SSD is a non-polar polystyrene-butadiene copolymer obtained from the Goodyear Tire and Rubber Company. The dye mixture was a solution as follows:

M1. (1) Eosin Y, Solvent Red No. 43, a 0.75% by weight solution in methanol, obtained from Matheson, Coleman and Bell 1.8 (2) Flurol 76A, Fluorescent Brightening Agent No. 75, a 1% methanol solution obtained from the General Dyestuff Co. 2.2 (3) Acetosol Blue GLS, Solvent Blue No. 46, a 1% toluene solution obtained from Sandoz, Inc. 1.4 i

A paper coated as above-described was stored in the dark at 50% relative humidity for sixteen hours and then processed. A uniformed electrostatic charge of about 650 volts was applied in about one-fourth of a second from a corona discharge device operating at 6 kv. potential. This charged coating was then exposed for about seven seconds to a light image to form an electrostatic latent image by discharging areas receiving radiation, and the latent image thus formed developed by contacting it with a liquid developer comprising finely divided colored toner particles suspended in an insulating liquid..The image was fixed by heat which removes the liquid by evaporation and fuses the toner. Exposure was for one second at a distance of /2-inch from a radiant heater at 450 F. Theprints had excellent contrast with uniform development of relatively large image areas. The recording element coatings above described had a charge acceptance of 650 volts in the dark, slow dark decay, and rapid charge dissipation, the rate of charge dissipation being 325 volts per second under exposure to an 88 foot candle source.

The currently preferred recording element is given in the following example.

EXAMPLE 2 Zinc oxide 208 Aropal 890x60 79 Pliolite SSD 14.5 Toluene 150 Sensitizing dye mixture ml 5.4

Aropal 890x 60 is a 60% solids dispersion of a polar styrenated-alkyd resin obtained from the Archer-Daniels- MidlandCompany. Coating and processing were in accordance with. Example 1 above.

EXAMPLE 3 Zinc oxide 54 Cycopol S101 7.5 Pliolite SSD 7 Sensitizing dye mixture ml 5.4 Toluene ml 38 EXAMPLE 4 This example was the same as Example 3 above excepting. that 10.5 grams of Cycopol S101 and 4.0 grams of Pliolite SD were used to. form the binder. Prints produced 4, by the procedure of Example 1 gave good contrast and uniform image development. Charge acceptance of the coating was 600 volts.

EXAMPLE 5 Zinc oxide 117 Pliolite SSD 10 Copolymer, vinyl acetate and 10% vinyl pyrrolidone Sensitizing dye mixture ml .54 Toluene 86 Paper coatings prepared and processed in accordance with Example .1, gave prints with good contrast and uni? form image development. The charge level acceptance was 400 volts with a rate of charge dissipation at 88 foot candle of 164 volts per second.

EXAMPLE 6 Zince oxide 58 Dow 276-V-2 3.6 Acyloid B82 11 Sensitizing dye mixture ml 5.4 Toluene 36 EXAMPLE 7 Zinc oxide 58 Resyl 750-17 10 Pliolite SSD 5 Sensitizing dye mixture "ml-.. 5.4 Toluene 38 Resyl 750-17 is a polar, talloil alkyl resin obtained from the American Cyanamid Company.

Coatings prepared and processed as described above provided prints with good contrast and uniform image development. Charge acceptance was 600 volts with a rate of charge decay at 88 foot candles with 200 volts per second.

The foregoing Examples 1-8 illustrate binders compris-.

ing mixtures of polar and non-polar film-forming resins.

Such mixtures are preferred, Example 2 above being the,

currently preferredcomposition. However, the required mixture having to a large degree the desirable characteristics herein described can also be prepared as a single copolymer. by copolmerizing polar and non-polar monomers as heretofore defined. This is illustrated in Example 9 below, styrene being the non-polar monomer and ethyl acrylate being polar.

EXAMPLE 9 Styrene 60 Ethyl acrylate (inhibitor free) 40 Toluene (C.P.) 150 Catalyst (azobisisobutyronitrile) 1.0

The toluene and 0.2 parts catalyst were charged into a 500 ml. flask equipped with stirrer, thermometer, N inlet, condenser and separatory funnel. The toluene was slowly heated to C. under N while adding, at the rate of one cc. per minute, the mixture of monomers and 0.6 part catalyst. Heating was continued for four hours at 95C., 0.2 part additional catalyst added, and heating then continued for four additional hours. The resulting 40% copolymer solution was cooled and used in preparing the 7 5 following coating.

Coating composition Copolymer solution 36 Zinc oxide 58 Additional toluene ml 25 sensitizing dye mixture ml 2 This composition was applied to paper and processed according to the procedure stated in Example 1. Charge acceptance was 580 volts and a dissipation rate with 88 foot candles illumination of 150 volts per second.

In addition to the resinous polar bindings indicated above, many other water-insoluble binder resins which are polar as defined herein can also be used. Examples include polyvinyl acetate, for example Vinylite AYAF of Union Carbide, and Chemicals Corporation; polyacrylic acid esters, for example C101V of the Rohm & H-ass Company; polystyrene-methacrylate copolymer, for example Zerlon of Dow Chemical Company; and toluene sulfonamide resins, such as Santolite MHP of the Monsanto Chemical Company. Addition-a1 non-polar binders include polystyrene, such as KTPL No. 6 of the Koppers Chemical Company, and various aromatic hydrocarbon resins, for example Panarez resins of the Amoco Chemicals Company.

While the present invention has been illustrated with the commercially preferred photoconductive zinc oxide as the photoconductive particles, it is well known that other photoconductive materials can be employed. It is further known that many dyes and dye mixtures in addition to the ones specified in the above examples can be used to sensitize the photoconductive material to the desired wavelengths of light. The 15 pounds per ream of coating weight suggested in the above example is merely preferred and various other coatings weights can be used if desired. A 15-pound coating weight provides a coating thickness between about one-half and one mil depending upon the coating procedure.

It should be further understood that the foregoing description is for the purpose of illustration, and the the invention includes all modifications falling within the scope of the appended claims.

What we claim is:

1. An electrostatic recording element for electrophotography comprising a support member coated with finely divided photoconductive particles suspended in an insulating film forming binder, said binder consisting essentially of a mixture of from about 33 to by weight of a polar, styrenated-alkyd resin and the remainder a nonpolar polystyrene-butadiene copolymer.

2. An electrostatic recording element for electrophotography comprising a supporting member coated with finely divided photoconductive particles suspended in an insulating film forming binder, said binder consisting essentially of a mixture of from about 33 to 80%, by weight, of a polar polyacrylic ester and the remainder, a non-polar poly-alpha-methylstyrene.

3. An electrostatic recording element for electrophotography comprising a supporting member coated with photoconductive particles suspended in an insulating film forming binder, said binder consisting essentially of a mixture of from about 33 to 80%, by weight, of a polar copolymer of vinyl acetate and vinyl pyrrolidone and the remainder, a non-polar polystyrene-butadiene copolymer.

References Cited UNITED STATES PATENTS 2,959,481 11/1960 Kucera 961 3,008,825 11/1961 Van DOrn et al 961 3,121,006 2/1964 Middleton et a1. 961 3,132,941 5/1964 Stahly et al. 961 3,152,895 10/1964 Tinker et al. 96-1 3,207,601 9/1965 Giaimo 961 3,245,786 4/1966 CaSsiers et al. 961

FOREIGN PATENTS 234,215 6/ 1961 Australia.

I. TRAVIS BROWN, Acting Primary Examiner.

NORMAN G. TORCHIN, Examiner.

C. E. VAN HORN, Assistant Examiner. 

1. AN ELECTROSTATIC RECORDING ELEMENT FOR ELECTROPHOTOGRAPHY COMPRISING A SUPPORT MEMBER COATED WITH FINELY DIVIDED PHOTOCONDUCTIVE PARTICLES SUSPENDED IN AN INSULATING FILM FORMING BINDER, SAID BINDER CONSISTING ESSENTIALLY OF A MIXTURE OF FROM ABOUT 33 TO 80% BY WEIGHT OF A POLAR, STYRENATED-ALKYD RESIN AND THE REMAINDER A NONPOLAR POLYSTYRENE-BUTADIENE COPOLYMER. 